1. Technical Field
This invention relates to a technique for reducing near-in sidelobes of an offset antenna, and more particularly, to a technique for reducing near-in sidelobes of an offset antenna which uses at least one piece of microwave absorbing material attached to the reflecting surface of the main reflector of the offset antenna, each piece bordering on a portion of the sector of the edge of the main reflector nearest the focal point of the main reflector.
2. Description of the Prior Art
Demands have been made in recent years to improve an antenna's sidelobe performance, since these sidelobes limit the number of converging routes of a common carrier microwave system. Far sidelobe reduction, for those sidelobes more than 35 degrees off the main beam, has been achieved for the horn reflector antenna by adding blinders (extensions to the sidewalls of the horn) to the aperture area of the antenna. While reducing the far sidelobes, this method does not improve the near-in sidelobe performance, where the most severe interference occurs.
Various methods have been tried to improve the near-in sidelobe performance, usually by coating the rim of the reflector with absorbing material. One example of such a method is disclosed in U.S. Pat. No. 3,314,071 issued to L. J. Lader et al on July 12, 1965. There, a tapered ring of absorbing material is attached to the periphery of the reflector. A reduction of the near-in sidelobes is achieved by the use of the tapered ring, but the phase error of the system is increased thereby, and the overall beamwidth must be increased to compensate for the increased phase error. Many of the attempts at sidelobe reduction using a peripheral ring of absorbing material suffer the same problems of increased phase error and the need for increased beamwidth.
Another approach, used with the horn reflector antenna, involves altering the horn to improve sidelobe performance. Like the sidelobe reduction methods described hereinabove involving the reflector, the horn is altered by additions of absorbing material. For example, it is known that a coating of absorbing material placed on the sidewalls of the horn is found to reduce the far sidelobes, but does not improve near-in sidelobe performance. An article in Electronics Letters, Vol. 9, No. 2, p. 26-27 by R. Ashton and R. Baldwin discusses a method of near-in sidelobe reduction using a dielectric slab placed across the aperture of a rectangular horn in the H field direction. The reduction is achieved, but at the expense of attenuation of the main beam of the radiation pattern due to the properties of the dielectric.
The problem remaining in the prior art is to provide a technique which permits selective reduction of near-in sidelobes with a minimal amount of attenuation of the main beam and increase in beamwidth.